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| DIY power supply |
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| MarkF:
The LM338 would work as well. The thread I pointed to has a few extra parts than normally shown in the datasheets. Here is another thread discussing bypass transistors: https://www.eevblog.com/forum/beginners/resistors-purpose/ That eBay 10-turn pot looks like the same Bourns one with different type pins. I rarely buy from eBay and put together larger orders with Mouser. You will have to go over the design for your voltage range and pick the pot value you will need. |
| Pirateguy:
do i need those transistors? if i am reading the datasheet correctly the 338 can do 5 amps, and that's about as much as i can imagine myself needing. i have a ZVS driver kit and a stungun module that ask for 12v, and those are the only times i have needed to account for amps beside limiting resistors for leds. if yes, anything with the right power capabilities will do? |
| MarkF:
It's not the current that is the limiting factor. It's the heat the LM338 will have to dissipate. 30W or more is going to generate a lot of heat. You will need a big heatsink and a fan. The worse case will be the voltage drop across the LM338 and the current drawn by the load. You can calculate the power the LM338 will need to handle by the worse case you expect to use. You could try with just a LM338 and see if it can handle power levels you want to use. Even with a PNP bypass transistor, you will be transferring (I should say sharing) the power dissipation from the LM338 to the transistor. The TIP36C transistor in the other thread comes in a TO-247 package which is almost twice the size of a TO-220. Much better to dissipate the heat. Most power supplies will have relays to switch to different input voltages so the regulators don't have to dissipate all the heat. |
| Pirateguy:
i was thinking of attaching them to the top of the big heatsink that's in there, which is right in line with the fan. what about the MJ2955? it is the compliment to the 2n3055 i have and comes in a to-3 package. also i have some ceramic plates to go in between the heatsink and transistor. |
| MarkF:
Let's not jump off the cliff just yet. Let's take a look at what you are trying to do here and what your options are. 1) Using the 32V 1.5A HP Supply, you could design something in the 0-30V @ 250mA to 500mA range without a bypass transistor. It would be a simple construction and in most cases it would be all you would need. For example, I have a 60V @ 250mA maximum supply. 2) Using the 32V HP Supply, you could add a bypass transistor to reach the 0-30V @ 1.5A range. Maybe limit to 1A to make it easier. A more involved build. 3) Using the 12V from the ATX Supply, you could get in the 0-10V @ 5A range. I think it would be rare that you would use these current levels. In the two cases you mentioned, you would used your 12V high current output directly from the ATX supply. To answer your question: You could build with a MJ2955. You could probably make almost any power PNP transistor work. You will need to adjust the resistor values to get the desired bias voltage. As for a variable power supply: Is this something you want to build or buy? How much effort do you want to do in the design and build? Personally, this is one item I would buy. Possibly with a LAN interface to automate some project testing. In reference to using the ATX heatsink. It is NOT something I would recommend. You have already compromised the cooling and airflow by removing the case. The saving grace is that you are NOT using anything near the capabilities of the supply. It is still to be seen if it gets hot during use. |
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